Study On The Motion Of Coarse Sediments In Oscillatory Flows

An important issue in coastal engineering is the bedload sediment transport in oscillatory flows,which is strongly related to some problems such as erosion of the coastal line and coastal structure scouring.Therefore,the characteristics of bedload sediment motion in oscillatory flows need to be studied and an accurate model to calculate the transport rate is in demand in order to help people predict short-term coastal line evolution,optimize the design of coastal engineering structures and prevent the scouring risks.In this thesis,coarse bedload sediment transport in oscillatory flows was studied via experiment in the oscillatory tunnel built by the author.The particle velocity and the number of moving particles in per unit area were measured to obtain the bedload sediment transport rate in each test.During the experiments,it was found that the velocity curve against time of the accelerating part was always flatter than that of the decelerating part,and there was a phase lag between the maximum transport rate and the maximum stream velocity.The two issues indicated the significance of inertia effect in coarse sediments motions.The existed quasi-steady model and semiunsteady model for medium and fine sediments failed to predict the experimental results accurately.Based on the force analysis of a single moving particle and the local equilibrium condition of erosion and accretion,the governing equations of the particle velocity and the number of moving particles in per unit area were derived,respectively.Then the instantaneous model for bedload sediment transport rate was established.The inertia force was taken into consideration in the new model.An explicit difference scheme was used to solve the governing equations.The calculated results fitted well with the measured results when the coefficients in the equations were given proper values.In particular,the models succeeded to simulate the phase lag phenomenon.A dimensionless parameter KC'' was introduced to describe the influence of inertia force on coarse bedload sediment motion.With the help of KC'',the momentum equation of sediment particles motion was simplified.Meanwhile,another formula to calculate the number of moving particles in per unit area was derived via energy balance analysis.Then a semi-analytical formula to calculate the half-cycle averaged bedload sediment transport rate was developed.The new formula consisted of two parts,the quasi-steady term and the unsteady term.Compared with the traditional quasi-steady formula,the new formula fitted better with the collected experimental data,especially when the KC'' was small.